don't know if its worth me posting this but I have in the past also had the same issue. Do a complete Kara run, HS to shat go to the T4 vendor to repair and then wait about 45secs for the game to unfreeze. Wondered what caused it, didnt think of whodrop.
Its only happened at that vendor so far though from memory (I aint tried the T5 vendor). Some other vendors lock me for about 2-3secs.
Not sure if latest versions have fixed it, I tend to avoid going to that vendor to repair now ;)
No sorry, have never run into an instance of WhoDrop causing lag when opening a window at a vendor.
Likely one of your mods that auto-sells. Or Nurfed. Or ColaMachine (the mod has been discontinued; it "Syncs the values received from a merchant scan", which may be causing the problem. Use ItemPriceTooltip instead.). Or TradeDispenser (hasn't been updated since January). Etc. etc.
Disable your addons 1/2 at a time (or one by one), and relog each time until you find the culprit.